Foaming of thermoplastic elastomers with different blowing agents and investigation of the effect on mechanical properties

In this study, the lightweighting of SBS polymer, which is widely used in various fields, particularly footwear and automotive, was investigated by foaming with physical and chemical blowing agents, along with the evaluation of changes in mechanical properties throughout the process. To optimize the rheological properties of SBS and ensure successful foaming, wax was incorporated into the SBS structure at different ratios (15%, 22.5%, 30%). The optimum wax concentration (30%) was determined, and the SBS polymer with a fixed wax content was used as the polymer phase in the subsequent stages of the study. For the blowing agents, experiments were conducted using one physical and two chemical blowing agents. Expanding microspheres, commonly known in the industry under the name Expancel, which consist of encapsulated hydrocarbons, were used as the physical blowing agent. Azodicarbonamide (AZDC) and sodium bicarbonate were used as chemical blowing agents. Additionally, catalysts—active zinc for AZDC and citric acid for sodium bicarbonate—were used to optimize the activation of chemical blowing agents. Granules ready for foaming were first prepared by incorporating different ratios of blowing agents and catalysts into the polymer structure using a single-screw extruder at temperatures below the activation point of the blowing agents. Subsequently, sheet-form products were obtained by compression molding at temperatures above the foaming threshold. The required raw material amounts for the mold cavities in the press were calculated to achieve a density of 0.6 g/cm³ during the process. Test specimens were cut from the obtained sheets according to appropriate standards, and tensile and abrasion tests were performed. The cellular structures of the samples were examined using an optical microscope. Additionally, the viscosity of the samples under extrusion conditions prior to foaming was determined using a capillary rheometer, and the effect of catalysts used for chemical blowing agents on the foaming temperature was analyzed using a rotasiyonal reometry. According to the results, the elastic modulus of the samples containing 3 phr Expancel as the physical blowing agent was determined to be 6.95 MPa. The elastic modulus of SBS samples containing 30% wax was measured as 4 MPa, but both samples retained an elongation capability of over 500%. In abrasion tests, the samples containing 3 phr Expancel and 30% wax demonstrated the best performance, with an abrasion coefficient of 536.12 and a Shore A hardness value of 54.